The A3 structure is the hexagonal close-packed structure. It
has an hexagonal Bravais lattice and two atoms per unit cell. 
When one atom is in the origin, the other is at (2/3,1/3,1/2) in crystal
coordinates. The position of the atoms is fixed by symmetry, while
the ratio c/a is an additional parameter.

The reference directory contains the following files:

asy_tmp.asy : An asymptote script to plot the Brillouin zone.

be_ph.exp : experimental inelastic neutron scattering data.
